[PPC] Fix assertion failure during binary encoding with -mcpu=pwr9

Summary
clang -c -mcpu=pwr9 test/CodeGen/PowerPC/build-vector-tests.ll causes an assertion failure during the binary encoding.
The failure occurs when a D-form load instruction takes two register operands instead of a register + an immediate.

This patch fixes the problem and also adds an assertion to catch this failure earlier before the binary encoding (i.e. during lit test).
The fix is from Nemanja Ivanovic @nemanjai.

Differential Revision: https://reviews.llvm.org/D33482

llvm-svn: 304133
diff --git a/llvm/lib/Target/PowerPC/PPCInstrInfo.cpp b/llvm/lib/Target/PowerPC/PPCInstrInfo.cpp
index 46f1031..fd6785e 100644
--- a/llvm/lib/Target/PowerPC/PPCInstrInfo.cpp
+++ b/llvm/lib/Target/PowerPC/PPCInstrInfo.cpp
@@ -1931,6 +1931,8 @@
   case PPC::DFSTOREf64: {
     assert(Subtarget.hasP9Vector() &&
            "Invalid D-Form Pseudo-ops on non-P9 target.");
+    assert(MI.getOperand(2).isReg() && MI.getOperand(1).isImm() &&
+           "D-form op must have register and immediate operands");
     unsigned UpperOpcode, LowerOpcode;
     switch (MI.getOpcode()) {
     case PPC::DFLOADf32: